Generally stated, hearing aids attempt to amplify the wide dynamic range of sounds found in the real world into the limited dynamic range of sounds that the impaired ear can hear. Crude hearing aids accomplish the need for different gain or volume levels by providing a manual volume control that may be manipulated by a user. More sophisticated hearing aids, however, use some form of automatic gain control or automatic volume control (xe2x80x9cAGCxe2x80x9d).
FIG. 1A illustrates a typical prior art AGC circuit. The circuit 101 comprises a variable gain amplifier 103 for adjusting the gain, an attack time constant resistor Ra 105, a diode 107, a release or recovery time constant resistor Rr 109, and a storage capacitor CS 111. In operation, as Vout increases, representing louder sounds, the diode 107 conducts, charging the storage capacitor CS 111. Charging of capacitor CS 111 in turn causes the variable gain amplifier 103 to reduce gain so Vout returns towards a fixed level. As Vout decreases, representing quieter sounds, the diode 107 no longer conducts, causing capacitor CS 111 to discharge through recovery resistor Rr 109. This, in turn, causes the variable gain amplifier 103 to increase gain so again Vout returns toward a fixed level. In other words, the ACG 101 increases gain for soft sounds or in the absence of sounds, and decreases gain for loud sounds.
While such a configuration was an improvement over manual volume/gain controls, it suffered from its own problems. For example, most hearing aids having an AGC circuit similar to that shown in FIG. 1A utilize fairly fast (e.g., approximately 10 mS) attack times and moderately fast (e.g., approximately 100-300 mS) release times. Shorter release times create noticeable distortion. If a longer release time is used, however, whole sentences may be missed following a sudden loud noise, such as, for example, a slamming door. Further, when the gain increases dramatically during quiet periods, the increased gain frequently causes feedback.
In addition, because of the very wide dynamic range of sounds in the real world and the very limited dynamic range of the impaired ear, high compression ratios are desirable. When combined with typical attack/release times mentioned above, however, high compression ratios cause a phenomenon known as xe2x80x9cpumping.xe2x80x9d More specifically, the background noise is amplified to a near normal level during brief pauses in speech, resulting in the user hearing word/noise/word/noise, etc. Prior art designs attempted to reduce the pumping effect by adjusting the threshold knee higher (above which the AGC is active), but resulted in mediocre, at best, results. In any case, such a prior art AGC design necessitates compromises among all the possible settings of compression ratio, attack time, release time and threshold knee.
One prior art attempt to improve over typical AGC designs, such as that shown in FIG. 1A, is to use two separate AGC circuits in series. The first AGC circuit is given fairly slow (e.g., approximately 1 second) attack/release times, and the second is given fairly fast (e.g., approximately 10 mS) attack/release times for signals at some level above the current level of the first AGC circuit. While this circuit is an improvement over typical AGC designs, it still suffers from the xe2x80x9cpumpingxe2x80x9d effect, just more slowly.
Another prior art attempt to improve over typical AGC designs is shown in FIG. 1B. As can be seen, the circuit 113 of FIG. 1B includes the same components of FIG. 1A, but adds a transistor Q1 115 in series with recovery resistor Rr 109. The input of the circuit 113 is connected to the base of the transistor Q1 115 through a preamplifier 117. In this configuration, the transistor Q1 115 acts as a switch with the base-emitter voltage as the reference. The base of transistor Q1 115 is driven by preamplifier 117 whose gain is a function of a feedback resistor Rr 119 and input resistor Ri 121. The values of these resistors are chosen to establish an output level of the preamplifier 117 equal to the base-emitter voltage of transistor Q1 115 when the input equals the desired threshold, typically 65 dB peak instantaneous.
In operation, for signals whose instantaneous amplified level exceeds the base-emitter voltage of the transistor Q1 115 (e.g., signals above 65 dB SPL), the transistor is on and the release or recovery time constant resistor Rr 109 is connected to ground. In this case, i.e., during the time the input is instantaneously above 65 dB SPL, for example, the circuit 113 of FIG. 1B acts like the conventional AGC circuit 101 of FIG. 1A. During the time that the amplified level is below the base-emitter voltage of the transistor Q1 115 (e.g., below 65 dB SPL), however, the release or recovery time constant resistor Rr 109 is disconnected from ground, causing the gain to be maintained at the most recent setting.
While the circuit 113 of FIG. 1B may arguably be an improvement over the circuit 101 of FIG. 1A, it still suffers from many of its own problems. For example, because the base-emitter forward bias voltage of the transistor Q1 115 performs the decision function, the threshold is ill defined. More specifically, the threshold is not a clear on/off characteristic, but rather an on/mostly-on/partly-on/partly-off/mostly-off/off characteristic. In other words, the circuit has multiple release time constants. Because the transistor is not fully on, recovery to soft speech is much slower than desired. Also, because the transistor is not fully off, undesired recovery occurs over a period of seconds. In other words, the transistor has a xe2x80x9cslushyxe2x80x9d threshold and requires large amounts of preamplifier gain to reach that threshold.
In addition, the very fast attack time and xe2x80x9cslushyxe2x80x9d recovery to soft speech creates an undesirable effect if a loud noise (e.g., door close, book slam, etc.) occurs in an environment of soft conversation. More specifically, the loud noise and fast attack time cause immediate and complete gain reduction, while a soft voice enables gain expansion only occasionally, causing the user to miss much of what is spoken.
It is therefore an object of the present invention to provide an improved AGC circuit for hearing aid and other related applications.
Other objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
An improved automatic gain control circuit for hearing aid and other related applications is provided. In one embodiment, the automatic gain control circuit or system is contained in a hearing aid. The hearing aid has an input transducer for converting sound energy into an electric signal. The hearing aid also includes control circuitry that receives the electrical signal and controls the gain of the electrical signal. In controlling the gain, the control circuitry uses only two release time constants, one representative of a gain control mode and the other representative of a gain adjust mode. A switch, which may be part of the control circuitry, is also included for switching between only the two time constants. The switch may, for example, select a short time constant if the amplitude of the electrical signal is greater than a predetermined threshold, and a relatively longer time constant if the amplitude of the electrical signal is less than the predetermined threshold. While this is the general case, selection of the longer release time constant may be delayed (i.e., the shorter release time constant may be retained) for a given period of time after the amplitude of the electrical signal falls below the predetermined threshold. Alternatively, the shorter release time constant may be selected for a period of time after the amplitude of the electrical signal rises above the predetermined threshold even if the amplitude of the electrical signal falls below the predetermined threshold during that period of time.
In any case, once the gain is set, an output transducer converts the gain controlled electrical signal into sound energy for transmission into the ear canal of a hearing aid user/wearer.
These and other advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.